1. Field of the Invention
The present invention relates to boundary acoustic wave devices used as resonators or filter devices, and more particularly to a boundary acoustic wave device having a three-medium structure in which a silicon oxide layer and a dielectric layer made of a dielectric having an acoustic velocity higher than that of silicon oxide are stacked on a piezoelectric substrate made of LiTaO3.
2. Description of the Related Art
Boundary acoustic wave devices have recently been attracting attention instead of surface acoustic wave devices. Boundary acoustic wave devices do not require packages having cavities, and therefore, the size thereof can be reduced.
International Patent Publication No. WO2007/124732 discloses a boundary acoustic wave device 101 having a sectional structure shown in FIG. 5. The boundary acoustic wave device 101 includes a piezoelectric substrate 102 made of LiTaO3 and first and second dielectric layers 103 and 104 stacked on the piezoelectric substrate 102 in that order. The second dielectric layer 104 has an acoustic velocity greater than that of the first dielectric layer 103. Interdigital transducer (IDT) electrodes 105 are provided on the upper surface of the piezoelectric substrate 102, and the first dielectric layer 103 is arranged so as to cover the IDT electrodes 105.
As described above, International Patent Publication No. WO2007/124732 discloses the boundary acoustic wave device 101 having a three-medium structure in which the first and second dielectric layers 103 and 104 are stacked on the piezoelectric substrate 102. Since the second dielectric layer 104 is provided, propagation loss in the boundary acoustic waves is reduced by the waveguide effect. In addition, in the piezoelectric substrate 102 made of LiTaO3, characteristics, such as an electromechanical coupling coefficient, can be increased by setting Euler angles within predetermined ranges.
However, according to the structure described in International Patent Publication No. WO2007/124732, the electromechanical coupling coefficient cannot be sufficiently increased even when the Euler angles are set within predetermined ranges.